This invention is in the field of mechanical cryogenic coolers, such as those using the Stirling cycle, and is particularly concerned with those coolers using a so-called split cycle. A good discussion of cryogenic refrigerators, including split cycle, appears in U.S. Pat. No. 3,630,041 of Dec. 28, 1971 to Danials et al. The split cycle refrigerator disclosed by Danials et al uses separate electrical motors for moving the hot and cold displacers. A more recent version of split cycle uses a pneumatically driven (free piston) cold displacers, as shown in U.S. Pat. No. 3,877,239 of Apr. 15, 1975 to Leo. An even more recent split cycle cooler is U.S. Pat. No. 4,092,833 of June 6, 1978 to Durenec. The problem exists in pneumatically driven cold displacers of what technique to employ in order to obtain a reliable and efficient return stroke for the cold displacer. The above-cited patents to Leo and Durenec respectively use a pneumatic spring and another compressor piston connected by another fluid line to the equivalent of Leo's pneumatic spring. Another technique for obtaining cold displacer return uses a pneumatic return cylinder with a seal having a controlled leakage from the displacer cylinder. All of these free piston systems have disadvantages compared to the instant invention. For example, the last mentioned technique above will vary in cylinder return timing as its seals undergo normal wear. Since the cited Durenec patent was issued, it has been found that a phase angle other than 180.degree. between pressure waves (spaces 19 and 20) in more desirable. In the cited Leo patent, seal wear will change piston timing. The instant invention is mechanically simpler than the Danials et al and the Durenec patents and allows for adjustment of displace piston timing after the device is assembled.